A square-wave brushless permanent-magnet DC motor is such a special brushless DC motor that the phase current and air gap magnetic field are approximately a square wave or a trapezoidal wave. For the three-phase bipolar driven brushless DC permanent-magnet motor, its forward conduction angle of each phase winding is 120°, pausing for 60°, then being on again backward for 120°, again pausing for 60°, and going round and round. The current of each phase winding is discontinuous, which renders current closed-loop control to be very difficult. Therefore, the current closed-loop control is seldom employed in the control system of the traditional square-wave brushless DC permanent-magnet motor.
In the prior art, the current closed-loop control is carried out by always applying an instantaneous value of the phase current to the three-phase square-wave brushless DC motor. This technical solution needs three independent current sensors as well as three independent current regulators, rendering the control circuit thereof to be complicated, difficult to adjust and poorly reliable, which thus is rarely adopted in the industry. There is a technical solution that uses an instantaneous value of bridge arm current in the prior art to realize the current closed-loop control. However, this technical solution of current sampling only approximately samples the current due to neglecting a freewheel function of motor winding inductance. Because freewheel current forms internal circulation in an inverting circuit and the motor winding, the bridge arm (bus) cannot be sampled, thus the actual current that flows through the motor winding and produces the torque cannot accurately be fed back, and thus the accurate control on the torque cannot be realized. It can be seen that this technical solution may result in a great intolerable deviation, and thus it is used to monitor a limited value of the current only.
On the other hand, current closed-loop control, speed closed-loop control and position closed-loop control are usually all essential in a high-performance servocontrol system. However, the prior technology has not realized the good current closed-loop control on the three-phase square-wave brushless DC motor. Therefore, in exiting high-performance servo control system, an AC servomotor or a sine brushless permanent-magnet DC motor is usually employed instead of the square-wave brushless permanent-magnet DC motor, which makes the control system significantly more complicated and the overall cost stay high.
Chinese patent application no. 200720118089.7 discloses “a brushless DC motor control system and the inverting module thereof”. In this application, the traditional inverting circuit is improved appropriately and a resultant current sensor is used to completely and continuously sample the three-phase current existing during the motor is on and performs current freewheel, thus the continuous closed-loop control can be performed on the three-phase current of the motor through a single current closed-loop regulator. However, the above-mentioned improvement is not a final and the simplest approach, and the present invention will make a further improvement based on it.